1. Field of the Invention
This invention relates to ethylene-propylene copolymers for foaming and a process for producing the same. More particularly, it relates to ethylene-propylene copolymers capable of producing foamed shaped articles having a structure of uniform and fine foams, even without adding any other resins such as polyethylene, EPR, etc.
2. Description of the Prior Art
Polypropylene is superior in the aspect of various physical characteristics such as heat resistance, chemical resistance, strengths, etc., but since the polymer has a low melt viscosity and a great temperature dependency of the melt viscosity, it has been regarded as difficult to obtain superior foamed products. In order to overcome such drawbacks of polypropylene, various processes have so far been proposed. For example, Japanese patent publication No. Sho 52-10149/1977 discloses a process of adding a high density polyethylene and an ethylene-propylene rubber to polypropylene; and Japanese patent publication No. Sho 52-22661/1977 discloses a process of adding a low density polyethylene to polypropylene. However, according to these processes, since other kinds of resins are added in a large amount, the cost becomes relatively high, and moreover, in oder to carry out uniform blending, powerful melt blending is required at the time of granulation into pellets to make it difficult to improve its productivity at low cost. Further, Japanese patent publication No. Sho 47-12864/1972 discloses a process of graft-polymerizing divinylbenzene onto polypropylene by means of radiation to improve melt viscosity, but since use of radiation is an indispensable requirement and the process takes a long time, it is difficult to regard the process as a general one. Japanese patent publication No. Sho 47-26170/1972 discloses a process of using an ethylene-propylene random copolymer, but since a random copolymer is used, it has the drawback that the high rigidity and heat resistance of polypropylene are damaged.
The present inventors have made strenuous studies in order to overcome various drawbacks of the above-mentioned prior art, and as a result have found that a block copolymer which consists of a propylene homopolymer portion and an ethylene-propylene copolymer portion; has definite differences between the molecular weights of the respective polymer portions in the molecule; and has a definite composition of ethylene and propylene, has much superior characteristics for foaming. (Note: "molecular weight" referred to herein means a weight of polymer portion or a weight of one polymer molecule.)
The object of the present invention is to provide an ethylene-propylene copolymer for foaming, as described above and a process for producing the same.